WO2020244963A1

WO2020244963A1 - An optical coupler

Info

Publication number: WO2020244963A1
Application number: PCT/EP2020/064542
Authority: WO
Inventors: Hugo Johan Cornelissen; Dirk Kornelis Gerhardus De Boer; Olexandr Valentynovych VDOVIN; Johannes Petrus Maria Ansems; Ludovicus Johannes Lambertus HAENEN; Barry Mos; Michel Cornelis Josephus Marie Vissenberg
Original assignee: Signify Holding B.V.
Priority date: 2019-06-04
Filing date: 2020-05-26
Publication date: 2020-12-10

Abstract

The present invention relates to an optical coupler for reshaping a light beam. The optical coupler comprises a first component in the form of a twisted rectangular rod having a rectangular light in-coupling surface and a rectangular light out-coupling surface. The first component is twisted such that the light out-coupling surface is rotated 90 degrees with respect to the light in-coupling surface. The optical coupler further comprises a second component in the form of a twisted rectangular rod having a rectangular light in-coupling surface and a rectangular light out-coupling surface. The second component is twisted such that the light out-coupling surface is rotated 90 degrees with respect to the light in-coupling surface. The light in-coupling surfaces of the first and the second components are arranged next to each other forming a rectangular common light in-coupling surface. The light out- coupling surfaces of the first and the second components are arranged next to each other forming a rectangular common light out-coupling surface. Each of the first component and the second component has an S-shaped twist rate. A lighting device comprising the optical coupler and a method for manufacturing the optical coupler are also presented.

Description

An optical coupler

FIELD OF THE INVENTION

The present inventive concept relates to an optical coupler suitable for coupling light between a light source and a light guide.

BACKGROUND OF THE INVENTION

An optical coupler is generally a device that may couple light between optical components. Optical couplers may be used in optical communication, imaging devices, telescopes, microscopes, lasers, etc. Currently, optical couplers are also used in various forms of general lighting fixtures such as in edge-lit lightguide based luminaires. Such luminaires distribute the light evenly over the area of the light guide for aesthetic and human centric effects and may be made thin and largely flat. Due to weight and material use concerns it is generally preferable to make light guides as thin as possible. Following the rapid

development of efficient white light-emitting diodes (LEDs), it is favored for use as a source of the light in luminaires and light fixtures. However, for efficient coupling of the light output by standard LEDs into thinner and thinner light guides, optical couplers may be required. Fig 1 A-D. Illustrates four examples optical couplers in the prior art. Fig. 1 A and Fig. IB show manifold optics for reshaping the light beam, with light shifters, to a different aspect ratio. These optics typically require the beam to be pre-collimated for low loss function. Fig. 1C shows a tapered coupler with tilted cylindrical faces. Such a device is inflexible with which types of LEDs and the number of LEDs that it may accommodate. Finally, Fig. ID shows a wedge-shaped coupler. Such a coupler may be provided with microgrooves on the widening faces of the wedge as to reshape the angular distribution of light and prevent losses from the tapering. However, these microgrooves may be difficult to form, and may require a more complex mold-based fabrication process. This is a common issue with the other examples in the prior art as well. Considering the above, it is clear that there is opportunity for improvement within the technical field. SUMMARY OF THE INVENTION

It is an object of the present invention to overcome at least some of the abovementioned problems.

According to a first aspect, an optical coupler for reshaping a light beam is provided. The optical coupler comprises a first component in the form of a twisted rectangular rod having a rectangular light in-coupling surface and a rectangular light out- coupling surface. The first component is twisted such that the light out-coupling surface is rotated 90 degrees with respect to the light in-coupling surface. The optical coupler further comprises a second component in the form of a twisted rectangular rod having a rectangular light in-coupling surface and a rectangular light out-coupling surface. The second component is twisted such that the light out-coupling surface is rotated 90 degrees with respect to the light in-coupling surface. The light in-coupling surfaces of the first and the second components are arranged next to each other forming a rectangular common light in-coupling surface. The light out-coupling surfaces of the first and the second components are arranged next to each other forming a rectangular common light out-coupling surface.

In the optical coupler according to the first aspect, each of the first component and the second component has an S-shaped twist rate, wherein the S-shaped twist rate of the first component may the same as the S-shaped twist rate of the second component.

By the wording“twisted” it is implied that the one longitudinal end of a component is rotated with respect to the opposite longitudinal end. The“optical coupler” may hereinafter also be referred to as the“coupler” for brevity.

The term“twist rate”, which may also be referred to as“rotation rate”, describes how subsequent cross sections of a component along the longitudinal direction of the component are rotated with respect to each other. When a plot of the rotation angle as a function of the location along the length of the component has the form of an S-shape, the component has an S-shaped twist rate.

The optical coupler may be used for reshaping a beam of light from the light in-coupling surfaces to the light out-coupling surfaces. The light is kept from escaping the coupler laterally by total internal reflection (TIR). TIR may be achieved by reducing the angle at which the light reflects against the lateral surfaces of the coupler.

Incident light, i.e. photons, having a sufficiently large angle between a directional vector of their propagation and a normal of the light in-coupling surfaces may accordingly be reflected at the lateral surfaces of the coupler and thus transverse the coupler longitudinally to the light out-coupling surfaces. The twisted components may improve light coupling efficiency, i.e. reduce light loss due to reflections at the lateral surfaces of the components. This may be attributed to the gradual twisting of the components, avoiding discontinuities suddenly presenting a larger angle for the light beam to reflect against. This reduces the need for pre-collimation, i.e. directing, of the light, adding extra complexity to the coupler. Furthermore, the need for micro optical elements to improve efficiency such as microgrooves may be reduced. The coupler may be modular and adapted for various types, sizes and numbers of light sources.

The provided optical coupler may be simpler to fabricate than the prior art optical couplers. The first and second components may e.g. be shaped as identical rectangular cuboids prior to twisting. Like in the prior art, molding, e.g. injection molding, may be used to create the components with relative ease. However, because of the simple geometry, extrusion may be used to form the components at a far accelerated rate, presenting a significant benefit in time saved and in increased fabrication throughput.

The respective rectangular light in-coupling surface of the first and second component may be non-quadratic. The respective rectangular light out-coupling surface of the first and second component may be non-quadratic.

By making the light out coupling surfaces of the components non-quadratic rectangles it is made possible to alter the aspect ratio, i.e. the ratio between the two dimensions of the rectangle, from the common in-coupling surface to the common out- coupling surface. By changing the aspect ratio of the surfaces, the aspect ratio of a light beam, transmitted through the coupler, may be accordingly altered. In the case with quadratic component coupling surfaces the aspect ratio may be rotated which may constitute altering the aspect ratio but without reducing the smallest dimension of the common surfaces.

A main advantage of non-quadratic component coupling surfaces is that they enable a reduction of the smallest dimension. This may in turn, enable more efficient coupling to thinner light guides or edge lit luminaires. Essentially, this may enable a thin edge-lit light guide to be efficiently provided with light from larger and more efficient standard light sources.

The rectangular light in-coupling surface of the second component may have the same dimensions as the rectangular light in-coupling surface of the first component.

The rectangular light out-coupling surface of the second component may have the same dimensions as the rectangular light out-coupling surface of the first component.

The rectangular light out-coupling surface of the first component may have the same dimensions as the rectangular light in-coupling surface of the first component. The rectangular light out-coupling surface of the second component may have the same dimensions as the rectangular light in-coupling surface of the second component.

Letting the couplings surfaces of a component be identical may mean the component is a cuboid or a rectangular cuboid prior to twisting. This enables fabrication by extrusion as described in the above. Identical coupling surfaces may also reduce the need for microgrooves of the lateral surfaces of the coupler, for low loss performance to be obtained.

The first component may be made out of an elastomeric material. The second component may be made out of an elastomeric material.

An advantage of elastomeric materials is that it may adapt elastically to other components moving or changing form, e.g. by thermal expansion. Edge-lit light guides tend to thermally expand during operation and it may therefore be preferable for the expansion to be accommodated for by elastic deformation of the optical coupler. During expansion, the coupler and the light guide may be kept in contact at the interface, preventing an increase in light loss due to discontinuities or gaps in the light path. Small deviations in shape from deformation may not result in noticeable leakage. Additionally, it may also be easier to bring an elastic coupler into contact with a light guide or a light source without using any additional bonding compounds. Using elastomeric materials for the components may further be preferable as it may enable easier twisting of the components and assembly of the coupler.

The common light in-coupling surface may have a width, A, and a height, B. The common light out-coupling surface may have a width 2A and a height 0.5B.

A may equal B.

Such dimensions imply a quadratic common light in-coupling surface. This is advantageous as many standard LED packages for general lighting are square shaped. The optical coupler discussed herein is therefore not reliant on special LED designs or packages to be supplied. The common light in-coupling surface may alternatively have a width, 2A, and a height, 0.5B with the common light out-coupling surface having a width A and a height B. Essentially this means utilizing the optical coupler reversed. Although not extensively discussed herein, this is also a relevant possibility if an elongated/thin beam or distribution of light needs to be reshaped to a squarer form.

The first component may be twisted clockwise. The second component may be twisted counter-clockwise.

Twisting the components in relatively opposing rotational directions may simplify fabrication of the coupler as the components may spatially overlap each other less and thus require less stretching of the material, or material tension, to fix the coupling surfaces. Alternatively, the first component may be twisted counter-clockwise and the second component may be twisted clockwise, i.e. vice versa to the above.

A further advantage of twisting the components in relatively opposing rotational directions may become apparent in the following, when discussing a hinge forming bridge portion.

The optical coupler may further comprise a hinge forming bridge portion. The hinge forming bridge portion may connect a corner edge of the first component and an adjacent corner edge of the second component along a longitudinal extension of the optical coupler extending from the common light in-coupling surface to the common light out- coupling surface.

This inclusion may be advantageous because it may simplify assembly of the two components post twisting. Otherwise assembly may entirely rely on glue or adhesive for fixing the coupling surfaces together to form the common coupling surfaces. A bridge portion may at least provide some extra mechanical stability to the coupler. A further advantage may be that the coupler may be formed in one part by e.g. extruding as will be discussed further in the following.

According to a second aspect, a lighting device is provided. The lighting device comprises a LED light source, a light guide that is thinner than a smallest width of the LED light source, and an optical coupler according to the first aspect. The optical coupler is arranged in between the LED light source and the light guide, with the common light in coupling surface facing the LED light source and with the common light out-coupling surface facing the light guide.

The“smallest width of the LED” may refer to the smallest dimension of a length and a width of a typical, rectangular chip shaped, LED package, wherein a thickness of the LED package, being a third dimension, is significantly smaller than the other two dimensions. The“light guide” may occasionally be referred to as the“edge-lit light guide”.

Generally, thinner edge lit-light guides are preferable for their reduced weight and the reduction in material requirements and costs for their fabrication. A typical value for light guide thickness may be 6mm. To address the above concerns, 4mm or 3mm thick light guides may be adopted. The thickness of the light guides may often be limited by the size of the LEDs. In general lighting, the standard LEDs used are mid-power LEDs that are typically 3mm x 3mm or larger. Other common alternatives are 3mm x 5mm or 5mm x 7mm. Smaller LEDs may be more expensive like high power LEDs. Smaller LEDs may also be less efficient such as smaller mid-power packages or highly asymmetric packages e.g. 1.5mm x 3mm. Smaller LEDs may additionally not contain sufficient flux. For example, two lines of low power LEDs may not produce enough flux for an edge-lit light guide general lighting luminaire.

Efficient and versatile optical couplers being able to reshape the light distribution to match the dimensions of thin light guides are therefore advantageous as they might lead to less light losses, due to the dimension mismatch, at the interface between the coupler and the light guide. A lighting device or system may present a good example of use for the optical coupler being a main object to this disclosure insofar. The discussed advantages related to less complex fabrication are still relevant as the optical coupler may relatively effortlessly be custom made or flexibly adapted to fit many sizes of light sources and light guides. Although an LED is the preferred light source, other sources of light may also be considered compatible.

According to a third aspect, a method for manufacturing an optical coupler according to the first aspect is provided. The method comprises extruding the first and second components, clockwise twisting the first component, and counter-clockwise twisting the second component.

The first and second components may be co-extruded.

As has been briefly discussed already, extruding may entail many advantages for fabrication of the optical coupler. Larger throughput may be the main advantage as components may be extruded in length and then cut into appropriate sizes as compared to injecting fluid polymer into a mold, cooling the polymer, and removing the product and repeating the process. Even if injection molding is used to form longer components, to be cut, or a plurality of components simultaneously, this approach still involves more complexity than extrusion. Injection molding e.g. may still require inflexible molds while an extruding device may only need to have its nozzle adapted.

The optical coupler or its components, may yet be formed by mold-based methods such as injection molding. In this case the procedure and the molds may be less complex than the discussed prior art. This may be attributed to a simpler macro scale geometry without micro scale grooves.

As an alternative to twisting, an extruder may be set up to create the twist of the components already during the extrusion step. This may be achieved by e.g. a longer and twisted extrusion nozzle. Twist-extruded components may be assembled with less

mechanical stress or tension. According to some embodiments the method comprises co-extruding the first component, the second component and the hinge forming bridge portion, clockwise twisting the first component, and counter-clockwise twisting the second component.

Forming the first and second components concurrently creates by co-extruding further reduces the fabrication efforts required and especially so in the case of co-extruding the components with the hinge forming bridge portion. This method may mean that all components and portions of the optical coupler are integrally formed. This may reduce efforts required to assemble two separate components to form the optical coupler.

A further scope of applicability of the present invention will become apparent from the detailed description given below. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.

Hence, it is to be understood that this invention is not limited to the particular component parts of the device described as such device may vary. It is also to be understood that the terminology used herein is for purpose of describing particular embodiments only, and is not intended to be limiting. It must be noted that, as used in the specification and the appended claim, the articles "a," "an," and "the," are intended to mean that there are one or more of the elements unless the context clearly dictates otherwise. Thus, for example, reference to "a lamp" or "the lamp" may include several devices, and the like. Furthermore, the words "comprising",“including”,“containing” and similar wordings does not exclude other elements or steps.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing embodiments of the invention. The figures should not be considered limiting the invention to the specific embodiment; instead they are used for explaining and understanding the invention.

Fig. 1 A illustrates a manifold optic coupler in the prior art.

Fig. IB illustrates another manifold optic coupler in the prior art.

Fig. 1C illustrates a tapered coupler with tilted cylindrical faces in the prior art.

Fig. ID illustrates a wedge-shaped coupler with microgrooves (not shown). Fig. 2 illustrates an optical coupler comprising two twisted components.

Fig. 3 A illustrates an optical coupler further comprising a hinge forming bridge portion in addition to the two twisted components.

Fig. 3B illustrates a cross sectional view at a light in-coupling surface.

Fig. 3C illustrates a cross sectional view between the coupling surfaces.

Fig. 3D illustrates a cross sectional view at a light out-coupling surface.

Fig. 4 illustrates a lighting device comprising a LED light source and a light guide in addition to the optical coupler.

Fig. 5 shows a flow chart for methods of manufacturing an optical coupler.

Fig. 6(a) shows a component in perspective view and intermediate cross sections of the component;

Fig. 6(b) shows the incoupling surface, the intermediate cross sections, and the outcoupling surface in a sequence from left to right;

Fig. 6(c) shows a graph wherein the rotation of a cross section is plotted as a function of the location of the cross section along the length of the component;

Fig. 7(a) shows a component in perspective view and intermediate cross sections of the component;

Fig. 7(b) shows the incoupling surface, the intermediate cross sections, and the outcoupling surface in a sequence from left to right;

Fig. 7(c) shows a graph wherein the rotation of a cross section is plotted as a function of the location of the cross section along the length of the component; and

Figs. 8(a) and 8(b) each shows a graphs wherein the efficiency is plotted as a function of the length for two different optical couplers.

As illustrated in the figures, the sizes of layers and regions are exaggerated for illustrative purposes and, thus, are provided to illustrate the general structures of

embodiments of the present invention. Like reference numerals refer to like elements throughout.

DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled person.

In Fig. 2, an optical coupler 100 for reshaping a light beam is illustrated. The optical coupler 100 comprises a first component 110 and a second component 120. Both components 110, 120 are in the form of a twisted rectangular rod. Both components 110, 120 also feature a respective rectangular light in-coupling surface 112, 122 and a respective rectangular light out-coupling surface 114, 124. The light in-coupling surfaces 112, 122 and the light out-coupling surfaces 114, 124 may be situated on opposite longitudinal ends of the respective components 112, 122. The first component 110 is twisted such that the light out- coupling surface 114 is rotated 90 degrees with respect to the light in-coupling surface 112. Similarly, the second component 120 is twisted such that the light out-coupling surface 124 is rotated 90 degrees with respect to the light in-coupling surface 122. The light in-coupling surfaces 112, 122 of the first and second components 110, 120 are arranged next to each other forming a rectangular common light in-coupling surface 132. The light out-coupling surfaces 114, 124 of the first and second components 110, 120 are arranged next to each other forming a rectangular common light out-coupling surface 134.

Prior to twisting, the first component 110 may be a hexahedron or a cuboid or a rectangular cuboid. Prior to twisting, the second component 110 may be a hexahedron or a cuboid or a rectangular cuboid. The first and second components 110, 120 may be substantially identical prior to twisting.

The first component 110 may be twisted clockwise and the second component 120 may be twisted counter-clockwise. This should be understood as the components 110,

120 being twisted in relatively opposite rotational directions to each other. The components 110, 120 or their longitudinal ends may also be translated to form the common coupling surfaces 132, 134.

The components 110, 120 may be formed by or at least comprise a light transparent material. Preferably, the material may have a refractive index in the range from 1.2-2, more preferably 1.4-1.8, and most preferably 1.55-1.65. The components 110, 120 may feature a refractive surface coating on lateral surfaces of the components 110, 120 that may increase the TIR. This may be attributed by the coating increasing the angle at which reflection may occur. The light in-coupling surfaces 112, 122 or the light out-coupling surfaces 114, 124, i.e. the longitudinal surfaces of the components 110, 120, may feature a surface coating. This coating may be different from the lateral surface coating and may be applied to match the refractive index of the components 110, 120 to the external media to which they are optically coupled. This coating may be applied to reduce losses at the interface between the components 110, 120 and the external media.

The components 110, 120 may be made out of a polymer material. Generally, this may be understood as the components 110, 120 being formed by or at least comprising polymer material. Some examples of polymer materials that may be used include

poly(methyl methacrylate) (PMMA), i.e. acrylic glass, polystyrene (PS), polycarbonate (PC), high temperature polycarbonate (HTPC), polyethylene terephthalate (PET), polyethylene naphthalate (PEN), cyclo olefin copolymer (COC), polyetherimide (PEI), polyarylate (PAR), polyphenylene sulfide (PPS), polyethersulfone (PES), polyetheretherketone (PEEK), polyimide (PI), polyamideimide (PAI), styrene acrylonitrile (SAN), acrylic copolymer, methyl, and mehylpentene.

The components 110, 120 may alternatively be made out of an elastomeric material. Generally, this may be understood as the components 110, 120 being formed by or at least comprising elastomeric material. Silicone rubber is an example of elastomeric material that may be used.

The rectangular light in-coupling surfaces 112, 122 of the components 110,

120 may be non-quadratic. The light out-coupling surfaces 114, 124 of the components 110, 120 may be non-quadratic.

The rectangular light in-coupling surface 122 of the second component 120 may have the same dimensions as the rectangular light in-coupling surface 112 of the first component 110. The rectangular light out-coupling surface 124 of the second component 120 may have the same dimensions as the rectangular light out-coupling surface 114 of the first component 110. The rectangular light out-coupling surface 114 of the first component 110 may have the same dimensions as the rectangular light in-coupling surface 112 of the first component 110. The rectangular light out-coupling surface 124 of the second component 120 may have the same dimensions as the rectangular light in-coupling surface 122 of the second component 120. Having the same dimensions may in general mean having a same surface area and a same, or a 90 degrees flipped, aspect ratio, between the length and height of the rectangular surface.

The common light in-coupling surface 132 may have a width, A, and a height, B, and the common light out-coupling surface 134 may have a width 2A and a height 0.5B. A and B are variables for linear distances. A may in some examples equal B. If this is the case, the common light-in coupling surface 132 may be shaped as a square, i.e. it will have an aspect ratio of 1 : 1. The common light out-coupling surface 134 may however have an aspect ratio of 4: 1 as the width 2A is four times as long as the height 0.5B=0.5A. Some examples of dimensions include a 4mm x 4mm, common in-coupling surface 132, i.e. A=4mm and B=4mm, and a 2mm x 8mm common out-coupling surface 134.

The common light in-coupling surface 132 may be configured as a point of entry for light from a light source. The light source may be adapted to match the dimensions of the common light in-coupling surface 132 and vice versa. The light source may be a LED.

The common light out-coupling surface 132 may be configured as a point of exit for light into a light guide. The light guide may be adapted to match at least one dimension of the common light out-coupling surface 134.

Simulations have shown that different materials may have different optimal dimensions. In general, longer optical couplers cause the optical coupler to approach the Fresnel limit for coupler efficiency, i.e. 91%. PS for example may be preferable for a shorter optical coupler 100 with a length shorter than 5 times the width, or height, of a square common light in-coupling surface 132. PMMA on the other hand may be preferable for a longer optical coupler 100 with a length longer than 5 times the width, or height, of the square common light in coupling surface 132. The length of the optical coupler 100 may be in the range of 5-50mm. The length range may preferably be 10-40mm, more preferably 15- 30mm, and most preferably 17-25mm.

In Fig. 3, the optical coupler 100 is shown to further comprise a hinge forming bridge portion 240. The hinge forming bridge portion 240 may connecting a corner edge 216 of the first component 110 and an adjacent comer edge 226 of the second component 120 along a longitudinal extension of the optical coupler 100 extending from the common light in-coupling surface 132 to the common light out-coupling surface 134.

The longitudinal extension may correspond to the direction of the propagation of light through the coupler 100. In Fig. 3 A, the bridge portion 240 is shown to follow the corner edges 216, 226 from the bottom of the coupler 100 at the light in-coupling surfaces 112, 122, 132 to the top of the coupler 100 at the light out-coupling surfaces 114, 124, 134. This is further illustrated by the cross-sectional views of Fig. 3B-D. Fig. 3B shows a cross section of the optical coupler 100 at the light in-coupling surfaces 112, 122, 132, Fig. 3D shows a cross section at the light out-coupling surfaces 114, 124, 134, and Fig. 3C shows a cross section between the light in-coupling surfaces 112, 122, 132 and the light out-coupling surfaces 114, 124, 134. The bridge portion 240 may comprise the same materials as the components 110, 120 that have been discussed in the above. The bridge portion 240 may thus comprise elastomeric material such as silicone rubber. In Fig. 4 a lighting device 300 comprising the optical coupler 100 is shown. The lighting device 300 further comprises a LED light source 302, a light guide 304 that is thinner than a smallest width of the LED light source 302. The optical coupler 100 is arranged in between the LED light source 302 and the light guide 304 with the common light in-coupling surface 132 facing the LED light source 302 and with the common light out- coupling surface 134 facing the light guide 304.

The LED light source 302 may be formed as a thin chip. The LED 302 may be a solid state, inorganic LED. The LED 302 may further be a blue LED, comprising at least GaN or InGaN semiconductor materials. The LED 302 may include doped semiconductor layers forming p-n junctions. The LED 302 may further comprise quantum wells or carrier blocking layers for increasing rates of radiative recombination. The LED 302 may also comprise a phosphor coating for light spectrum modulation. The phosphor coating may be specifically adapted to modulate the predominantly blue light output to a broader spectrum of essentially white light. Using a plurality of LEDs of different emission wavelengths may also be considered for forming white light by light mixing. An example of such a device may include a red, green and blue (RGB) LED. Regardless of the LED type and any chooses scheme for light modulation, a plurality of dependent or independent LEDs 302 may be considered instead of using a single LED 302.

The LED 302 may further comprise a package including electrical connections and an encapsulation layer. The LED light source 302 may be a top-emitting LED, meaning that it is arranged to emit light mainly in the top direction. The LED 302 may alternatively be a side-emitting LED. If the LED 302 in Fig. 4 is a top-emitting LED, it may emit a large portion of the total light emission output into the optical coupler 100 via the light in-coupling surfaces 112, 122, 132.

The dimensions for height and width of the LED 302, i.e. not its thickness, may generally be in the range of 1-lOmm, preferably 1.5-8mm, more preferably 2-6mm thick, and most preferably 2.5-4.5mm. The LED 302 may preferably be square shaped i.e. quadratic. The optical coupler 100 may be adapted accordingly, to feature the same or near same dimensions at the light in-coupling surfaces 112, 122, 132. A typical example of LED 302 dimensions may be 4 x 4mm.

The light guide 304 may be an edge-lit light guide for a luminaire or light fixture. The light guide 304 may comprise the same materials as the components 110, 120 that have been discussed in the above including elastomeric materials such as silicone rubber. The light guide 304 may also comprise glass or glass-based materials. The light guide 304 may also comprise surface coatings as described for the components 110, 120 in the above. Refractive coatings to the large top and bottom sides may be particularly appropriate in order to increase TIR of e.g. an edge-lit light guide. The light guide 304 may further comprise at least one diffusion element that may intentionally provide the internally reflecting light with a path to escape the light guide and thus illuminate a surrounding. Such a diffusion element may be realized by forming a dimple or other types of irregularities on an inner surface of the light guide 304.

The light guide 304 may generally be 1-lOmm thick, preferably 1.5-8mm thick, more preferably 2-6mm thick, and most preferably 2.5-4.5mm thick. The optical coupler 100 may be adapted accordingly, to feature a same or a near same thickness at the light out-coupling surfaces 114, 124, 134. The other spatial dimensions of the light guide 304 may vary. A small light guide 304 may be adapted to be in connection a single optical coupler 100 and receive light, via the coupler 100, from a single LED 302. This is basically what is illustrated by Fig. 4. A large light guide 304 may also be considered with a large number of LEDs 302 and optical couplers 100 providing it with light.

The light guide 304 may be adapted as a lighting panel for illumination of a room. The panel may be integrated with the ceiling. The light guide 304 may furthermore be used in many other luminaires and light fixtures.

In Fig. 5, a flow chart for methods of manufacturing the optical coupler 100 are shown. The step of extruding S501 of the first and second components 110, 120 may be performed by an extruder. The extruder may comprise a barrel chamber, a screw, means for driving the screw, i.e. a motor, a nozzle portion, a feeding portion and heating elements. The elements 110, 120 may be formed as cuboid structures formed by extruding material, e.g. polymers discussed in the above, through a rectangular nozzle of the extruder. The components 110, 120 may be co-extruded. The extrusion may be performed at elevated temperatures, i.e. higher than room temperature.

Post extrusion the components 110, 120 may be cooled or allowed to return to room temperature if elevated temperatures were used during extrusion. The first component 110 may then be twisted S503a clockwise. The second component 120 may be twisted S503b counter-clockwise. The twisting may be performed such that the components 110, 120 are rotated substantially 90 degrees. Twisting may be performed such that the light in-coupling surfaces 112, 122 are located adjacent to each other such that they may form the common light in-coupling surface 132. Twisting may be performed such that the light out-coupling surfaces 114, 124 are located adjacent to each other such that they may form the common light out-coupling surface 134. Alternatively, the components 110, 120 may be twisted at an elevated temperature as to let them cool down and harden with a stable twisted shape.

The twisting may entail elastic deformation, in which case the components 110, 120 may require fixing by e.g. adhesive or glue their longitudinal ends. The fixing may utilize clamping means to mechanically fix the ends while the adhesive or glue cures or hardens. Alternatively, the components 110, 120 may be fixed mechanically long-term by an added mechanical fixing component. The twisting may entail plastic deformation of the material of the components 110, 120. Such twisting may require less effort to fix the components 110, 120 as the material may not be as inclined to flex back to its original form or shape.

As an alternative to the step of extruding S501 the first and the second components 110, 120 as individual parts, an optical coupler 100 may be fabricated by co extruding S502 the first component, the second component 120, and the hinge forming bridge portion 240, followed by the steps of clockwise and counter-clockwise twisting S503a,

S503b. The extrusion may in this case be performed through a V-shaped nozzle, matching the cross-section of Fig. 3C. This may improve extrusion as the components 110, 120 are extruded with as much spacing between them as possible considering that the comer edges 216, 226 need to be extruded adjacent to each other with the hinge forming bridge portion 240 connecting the two. A V-shape may also require a less complex and more robust extrusion nozzle with less cantilevering separation elements. The components 110, 120 of a V-shaped coupler 100 may be twisted 45 degrees, in opposite directions at the ends of each component, to form the common coupling surfaces 132, 134.

In the optical coupler according to the invention, each of the first component 110 and the second component 120 has, along its length, a so-called twist rate, which may also be referred to as a rotation rate. The twist rate describes how subsequent cross sections of the component along the longitudinal direction of the component are rotated with respect to each other.

Figure 6(a) shows a component 600 in perspective view. The component has a rectangular incoupling surface 610 and a rectangular outcoupling surface 620. The outcoupling surface 620 has the same dimensions as the incoupling surface 610. The outcoupling surface 620 is rotated over 90 degrees in clockwise direction compared to the incoupling surface 610. Figure 6(a) also shows intermediate cross sections of the component 600 at regular intervals between the incoupling surface 610 and the outcoupling surface 620. In a sequence from left to right, Figure 6(b) shows the incoupling surface 610, the intermediate cross sections at regular intervals, and the outcoupling surface 620.

Figure 6(c) shows a graph wherein the rotation of a cross section is plotted as a function of the location of the cross section along the length of the component 600. The numbers 1 to 10 correspond to the incoupling surface 610 (number 1), the intermediate cross sections (numbers 2 to 9) and the outcoupling surface 620 (number 10). Because the graph is a straight line, the component 600 is said to have a linear twist rate (or rotation rate).

Similar to Figure 6(a), Figure 7(a) shows a perspective view of a component 700 with in- and outcoupling surfaces 710 and 720, respectively, that are both rectangular and have the same dimensions. The outcoupling surface 720 is rotated over 90 degrees in counter-clockwise direction compared to the incoupling surface 610. Figure 7(a) also shows intermediate cross sections of the component 700 at regular intervals between the incoupling surface 710 and the outcoupling surface 720.

Similar to Figure 6(b), Figure 7(b) shows, in a sequence from left to right, the incoupling surface 710, the intermediate cross sections at regular intervals, and the outcoupling surface 720.

Similar to Figure 6(c), Figure 7(c) shows a graph wherein the rotation of a cross section is plotted as a function of the location of the cross section along the length of the component 700. The numbers 1 to 10 correspond to the incoupling surface 710 (number 1), the intermediate cross sections (numbers 2 to 9) and the outcoupling surface 720 (number 10). In contrast to the graph of Figure 6(c), the graph of Figure 7(c) is not a straight line. Instead, it is an S-shaped curve. Therefore, the component 700 is said to have an S-shaped twist rate (or rotation rate).

Along its length direction, a component with an S-shaped twist rate initially exhibits a less rapid rotation and finally a more rapid rotation as compared to a component with a linear twist rate.

Examples of S-shaped curves are curves of logistic functions (i.e. sigmoid curves), arctangent functions and hyperbolic tangent functions. Certain algebraic functions may also give S-shaped curves.

Simulations have shown that an S-shaped twist rate improves efficiency. This is illustrated in Figure 8, showing two graphs, in each of which the efficiency is plotted as a function of the length for two different optical couplers: (i) a first optical coupler with two components, each being of the type shown in Figure 6(a), i.e. having a linear twist rate, and (ii) a second optical coupler with two components, each being of the type shown in Figure 7(a), i.e. having an S-shaped twist rate. Both optical couplers have a rectangular common light-incoupling surface of 4 by 4 millimeters and a rectangular common light-outcoupling surface of 8 by 2 millimeters.

In the graph of Figure 8(a), both optical couplers are made of PMMA. In the graph of Figure 8(b), both optical couplers are made of PS.

From the graphs shown in Figures 8(a) and 8(b) it is clear that, particularly for relatively short lengths, the efficiency of an optical coupler having two components with (the same) S-shaped twist rate is higher than the efficiency of an optical coupler having two components with linear twist rates.

An optical coupler is said to have a relatively short length if it has a length that is less than 5 times the shortest of the width and height of its common light in-coupling surface. For example, for optical couplers having a common light-incoupling surface of 4 by 4 millimeters, a relatively short optical coupler would be an optical coupler with a length of 20 millimeters or less.

From Figure 8(a) it can be seen that for an optical coupler made of PMMA with a common light-incoupling surface of 4 by 4 millimeters and with a length of 10 millimeters, the efficiency is 81 % in case of a linear twist rate and 84 % in case of an S- shaped twist rate, which corresponds to an efficiency gain of 4 % (i.e., a factor of 1.04).

The efficiency gains illustrated in the graphs of Figures 8(a) and 8(b) are not limited to the specific optical couplers that are made of PMMA and PS. Similar efficiency gains are achieved for optical couplers that are made from other materials, with other common light in- and outcoupling surfaces, and with other S-shaped twist rates.

The higher the lumen density that is required for a certain application, the more important an efficiency gain becomes. For high lumen density (HLD) applications an efficiency gain of even a few percent would already be highly significant.

HLD applications are typically based on LED- or laser-pumped luminescent rods. These rods in general comprise a high-refractive-index material, such as lutetium aluminum garnet (LuAG) and yttrium aluminum garnet (YAG), having a refractive index of 1.83.

HLD applications typically make use of optical elements for extracting light and shaping light into a desired angular and spatial distribution. An example of a suitable optical element is a compound parabolic concentrator (CPC). The optical element used for extraction and shaping is preferably as efficient as possible. To maximize light extraction, the optical element may be arranged in optical contact with the luminescent rod. Leakage of light from the sides of the optical element may be prevented, for example by having a relatively high refractive index and/or by having a shape that is curved such that no angles outside total internal reflection (TIR) occur. When the optical element has a refractive index that is substantially equal to the refractive index of the luminescent rod, substantially all light will be extracted and substantially no light will leak, because the light will all be totally internally reflected.

In an HLD application, one may use a combination of a first optical element and a second optical element. The first optical element may be a light extracting and/or light concentrating optical component, such as a CPC, with a relatively high refractive index. If the first optical element is designed to provide a light distribution with an angle of 90 degrees, no light leakage due to loss of total internal reflection will occur if the refractive index of the second optical element is larger than the square root of 2 (in other words, when the refractive index of the second optical element is larger than 1.41). In case the second optical element is curved such that no angles outside TIR occur, the second optical element may have a lower index of refraction, such a refractive index of 1, meaning that a hollow reflective CPC may be used as second optical component.

If the second element is curved such that no angles outside TIR occur, the first optical element may be designed to provide a light distribution with an angle less than 90 degrees.

In an HLD application, one may also use the optical coupler according to the present invention, particularly for the purpose of aspect ratio conversion.

For example, an HLD application may have a luminescent rod with a light exit surface having an aspect ratio of 4: 1. Coupled to the light exit surface of the luminescent rod is a first optical element in the form of a CPC comprising a material with a relatively high refractive index, wherein the CPC is designed for receiving a light distribution with an incoming angle of 90 degrees in the material with a relative high refractive index and for providing a light distribution with an outgoing angle of 90 degrees in air. In this example, the CPC is a rectangular CPC with a long side having the shape of a CPC and a short side having the shape of a stretched CPC with a stretch factor 4. An optical coupler according to the present invention is used as a second optical element, for changing the aspect ratio to 1 : 1.

The optical coupler is made of a material with a relatively low index of refraction, such as an index of refraction of 1.45. A square CPC is, made of the same material as the second optical element, is used as a third optical element. The third optical element is designed for receiving a light distribution with an incoming angle of 90 degrees in air and for providing a light distribution with an outgoing angle of 34 degrees in air. For the optical coupler used as a second optical element, the length and the common light out-coupling surface has a ratio of 5.5, which results in a light leakage of about 3 % in the optical coupler. The amount of light leakage may be lowered by using a longer optical coupler.

In the above example, the third optical element may be omitted. If this is done, the resulting light distribution would have a width of approximately 90 degrees. Some light entering at large angles at the end surfaces of the optical coupler would be reflected by TIR, which results in a somewhat reduced efficiency.

Also, when in the above example the third optical component is omitted, the optical coupler may have a common light out-coupling surface that is larger that the common light in-coupling surface. If this is the case, the optical coupler also enables collimation of light.

An example of an optical coupler wherein the common light out-coupling surface is larger that the common light in-coupling surface has a common light in-coupling surface of 2 by 8 millimeters (formed by two light in-coupling surfaces of first and second components of 2 by 4 millimeters each), and a common light out-coupling surface of 7.2 by 7.2 millimeters (formed by two light out-coupling surfaces of first and second components of 3.6 by 7.2 millimeters each). In this case, the ratio of the common light out-coupling and light in-coupling surfaces is such that an incident light distribution with angles up to 90 degrees is converted into an outgoing light distribution with angles up to 34 degrees. In other words, the optical coupler is arranged to reduce the angular spread of the light. Depending on the length of the optical coupler, the light distributions provided by the first and second components may be shifted relative to each other. Such a shift becomes smaller for longer optical couplers.

In the above example, the size dimensions of the optical coupler may change linearly with distance, or they may change non-linearly with distance. For the latter, the size dimensions by be changed by parametrizing the shape using a Bezier-curve description. This could for example result in a concave (or trumpet-like) shape, which may give a more symmetric angular distribution for a given length of the optical coupler, and which may ensure that more light can be collected within a desired cone.

As mentioned above, an optical coupler according to the present invention, wherein the common light out-coupling surface is larger that the common light in-coupling surface, may be used in an HLD application. Alternatively, it may be used in combination with a light source (such as an LED-based light source) that has a different aspect ratio than the desired application. It may be especially favourable in combination with a relatively thin and broad luminescent rod. The cooling of such a rod would be much easier than that of a rod with a cross section that is approximately square. Moreover, it would be much easier to couple blue light into such a relatively flat luminescent rod.

To increase efficiency of the optical coupler according to the invention, a light redirecting prism may be provided adjacent to the common light-outcoupling surface. Such a light redirecting prism may correct any skewness of the angular light distribution to provide a centered light distribution. The light redirecting prism may be replaced by an array of micro prisms, such as prisms with a base of 0.5 by 0.5 millimeters and a height of 0.33 millimeters, wherein each micro-prism has a similar shape as the light redirecting prism.

Further to the above, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

Claims

CLAIMS:

1 An optical coupler (100) for reshaping a light beam, the optical coupler comprising:

a first component (110) in the form of a twisted rectangular rod having a rectangular light in-coupling surface (112) and a rectangular light out-coupling surface (114), wherein the first component is twisted such that the light out-coupling surface is rotated 90 degrees with respect to the light in-coupling surface; and

a second component (120) in the form of a twisted rectangular rod having a rectangular light in-coupling surface (122) and a rectangular light out-coupling surface (124), wherein the second component is twisted such that the light out-coupling surface is rotated 90 degrees with respect to the light in-coupling surface;

wherein the light in-coupling surfaces of the first and the second components are arranged next to each other forming a rectangular common light in-coupling surface (132),

wherein the light out-coupling surfaces of the first and the second components are arranged next to each other forming a rectangular common light out-coupling surface (134), and

wherein each of the first component (110) and the second component (120) has an S-shaped twist rate.

2. The optical coupler according to claim 1, wherein the respective rectangular light in-coupling surface of the first and second component is non-quadratic and wherein the respective rectangular light out-coupling surface of the first and second component is non quadratic.

3. The optical coupler according to any one of claims 1-2, wherein the rectangular light in-coupling surface of the second component has the same dimensions as the rectangular light in-coupling surface of the first component.

4. The optical coupler according to any one of claims 1-3, wherein the rectangular light out-coupling surface of the second component has the same dimensions as the rectangular light out-coupling surface of the first component.

5. The optical coupler according to any one of claims 1-4, wherein the rectangular light out-coupling surface of the first component has the same dimensions as the rectangular light in-coupling surface of the first component.

6. The optical coupler according to any one of claims 1-5, wherein the rectangular light out-coupling surface of the second component has the same dimensions as the rectangular light in-coupling surface of the second component.

7. The optical coupler according to any one of claims 1-6, wherein the first and the second components are made out of an elastomeric material.

8. The optical coupler according to any one of claims 1-7, wherein the common light in-coupling surface has a width, A, and a height, B, and wherein the common light out- coupling surface has a width 2A and a height 0.5B.

9. The optical coupler according to claim 8, wherein A=B.

10. The optical coupler according to any one of claims 1-9, wherein the first component is twisted clockwise and wherein the second component is twisted counter clockwise.

11. The optical coupler according to any one of claims 1-10, further comprising a hinge forming bridge portion (240) connecting a corner edge (216) of the first component and an adjacent corner edge (226) of the second component along a longitudinal extension of the optical coupler extending from the common light in-coupling surface to the common light out-coupling surface.

12. A lighting device (300) comprising:

a LED light source (302);

a light guide (304) that is thinner than a smallest width of the LED light source; and

an optical coupler (100) according to any one of claims 1-11; wherein the optical coupler is arranged in between the LED light source and the light guide, with the common light in-coupling surface (132) facing the LED light source and with the common light out-coupling surface (134) facing the light guide.

13. A method for manufacturing an optical coupler (100) according to claim 1, wherein the method comprises:

extruding (S501) the first and second components (110, 120); clockwise twisting (S503a) the first component; and

counter-clockwise twisting (S504b) the second component.

14. The method according to claim 13, wherein the first and second components are co-extruded.

15. A method for manufacturing an optical coupler (100) according claim 11, wherein the method comprises:

co-extruding (S502) the first component (110), the second component (120) and the hinge forming bridge portion (240);

clockwise twisting (S503a) the first component; and

counter-clockwise twisting (S503b) the second component.